1. Field of the Invention
The present invention concerns a water-dispersible oil composition for hot rolling aluminum and aluminum alloys, which has excellent rolling lubricity and emulsion stability, and which provides a rolled sheet with an excellent surface quality.
2. Discussion of the Background
In the process of hot rolling aluminum and aluminum alloys, a so-called roll coating is formed on the surface of the rolls as a result of the transfer of aluminum from the surface of the rolled plate to the surface of the rolls. If the rolls have a roll coating thereon, a billet is rolled while in contact with the roll coating. This means that the surface quality of the rolled plate varies according to the properties of the roll coating, and defects in the surface affect the quality of the plate surface after cold rolling. Thus, the nature of the roll coating is very important. It varies depending on the rolling conditions (materials to be rolled, temperature of the plates, roughness of the plate surface, temperature of the rolls, roughness of the roll surface, draft, rolling speed, brush roll operating conditions, etc.) and the kinds of rolling oils. Accordingly, selection of a rolling oil is essential in the control of roll coatings.
In hot rolling, rolling oils are used in the form of an emulsion since sufficient roll cooling properties are required. Conventional hot rolling oils for aluminum and aluminum alloys are emulsions having concentrations of 3 to 10% of oil components. They are generally prepared by incorporating oil performance improvers; extreme pressure agents; anti-rusting agent; anti-oxidants; etc. into mineral oil as base oil and emulsifying the resulting mixture, primarily with anionic surfactants. Oil performance improvers may include fatty acids, oils and fats, and fatty esters.
Desirable features for oils for hot rolling aluminum and aluminum alloys include lubricity, good roll coating properties, good surface quality, emulsion stability, workability, and waste treatment properties. Especially in recent years, the need to carry out mass production and the desire for high quality in rolled aluminum products have resulted in the demand for hot rolling oils having high levels of lubricity, surface quality and emulsion stability. However, conventional oils for hot rolling aluminum and aluminum alloys do not sufficiently meet all these requirements.
In conventional rolling oils, lubricity has been controlled by changing the kinds and amounts of emulsifiers. However, in conventional hot rolling oils which contain emulsifiers, lubricity and emulsion stability tend to clash, and both properties typically cannot be met at the same time. Namely, when the lubricity increases, the emulsion stability decreases, causing deteriorated lubrication stability over time. Likewise, when the emulsion stability is enhanced, sufficient lubricity cannot be obtained. Eventually, the plate surface develops defects. Therefore, constant quality of rolled plate surfaces over time generally are not obtained with conventional rolling oils.
Japanese Patent Publication (kokoku) No. 14599/1987 discloses a rolling oil composition in which the lubricity and emulsion stability are both satisfied. Although the technique of this publication is unique and successful in uniting lubricity and emulsion stability, the surface quality of the obtained rolled plates is not necessarily satisfactory. Japanese Patent Application Laid-open (kokai) No. 120795/1988 discloses a similar approach. The technique of this publication is essentially the same as disclosed in Japanese Patent Publication (kokoku) No. 14599/1987, but this publication improves the surface quality by sacrificing lubricity.
Thus, in conventional oils for hot rolling aluminum and aluminum alloys, satisfactory levels of lubricity, emulsion stability and surface quality typically are not provided simultaneously. Therefore, conventional oils are generally unsuitable for mass production of aluminum or aluminum alloys in present-day methods of rolling, and have difficulty in meeting the desire for rolled products of high quality.
In view of the foregoing, the present inventors found that good lubricity, emulsion stability and surface quality can be simultaneously obtained by emulsifying and dispersing a specified lubricant component in water using a specified polymer compound. A patent application (Japanese Patent Application Laid-open (kokai) No. 145692/1990) has been filed, directed towards these findings. However, further research revealed that this hot rolling oil composition is not free from heat degradation after long term use under severe hot rolling conditions. Such heat degradation deteriorates the performance of the composition and the surface of the rolled aluminum or aluminum alloy.
As a consequence, a need still exists for an oil composition which, when emulsified in water, provides satisfactory levels of lubricity, emulsion stability and surface quality simultaneously over a prolonged period of time.